Shells are

all along the nearby shoreline. Modiolus modiolus

Class: Bivalvia Order: Mytiloida Family: Mytilidae Genus: Modiolus

Distribution Its global range includes the coasts of Japan, Iceland, Europe, This species is widely north-western Africa and the Mediterranean Sea. It occurs on distributed in the northern the European seaboard of the Atlantic Ocean from the United hemisphere. It is found Kingdom northwards. It occupies all of the North Sea and along the Atlantic coast of extends south to the Bay of Biscay. There are large beds of these North America, from the mussels along the coast of Scotland. Arctic Ocean to Florida, They occur on both the west and east coasts of Canada but are and along the Pacific coast, far greater in number all along the eastern seaboard. They from the Arctic Ocean to extend from Nova Scotia up into the Arctic Ocean. California. It can be seen singly usually in rough ground but also in huge Habitat beds. It can be found on the lower shore in rock pools or in The horse mussel is found laminarian (seaweed) holdfasts but more common subtidally to partly buried in soft depths approaching 300m. It is found from low tide mark to sediments or coarse depths of 50 metres in British waters and 80 metres off the coast grounds or attached to hard of Nova Scotia. Individuals have been found at depths of up to substrata, forming clumps 280 m. Water movement appears to be an important factor in or extensive beds or reefs. the build up of many of the denser reef areas, the majority being found in areas of moderate to strong tidal currents. Food They are suspension filter Food availability for suspension feeders is linked to seasonal feeders. Particles of varying production. Adult M. modiolus in Nova Scotia occur commonly size are extracted from in areas with moderate to high water exchange. They ingest water as it is drawn into the phytoplankton, detritus, bacteria, and non-living organic matter. body cavity. Feeding material is sorted by size and rejected particles fall off gill edges as pseudo-feces. Reproduction Mussels have separate The timing and duration of gametogenesis and spawning varies a sexes. Sperm and eggs are great deal globally. Fertilization is in response to environmental released into the water conditions. Depth and latitude (water temperature) are column for fertilization. contributing factors. In this species mussels are long lived and Spawning peaks in spring slow to reproduce. They do not become sexually mature and and summer. capable of reproduction for several years.

Development Settling larvae metamorphose into young Fertilized eggs become juveniles and proceed to find a primary larvae. Larval development settlement location. They prefer the is planktotrophic, with a byssus threads and aggregations or short trochophore stage clumps of adults which provide a refuge prior to the veliger stages. from predators. Juveniles growing on They are free swimming for byssus threads are more likely to survive three to four weeks. During than free living individuals. These newly- this time they develop settled young mussels exhibit rapid several structures, including growth in the first four to six years prior a pair of ocelli and a foot, to reaching maturity. They invest energy retained through life. in growth rather than reproduction.

Characteristics. Young shells have numerous long, smooth, spines which wear This is a large, robust away in the adult. The interior of the shell is white, with a faint mussel with the potential to bluish tinge. It has a wide pallial line, a large anterior adductor grow to 22 cm or more. 10 muscle scar and small posterior adductor muscle scar. The two cm, however, is typical. Size valves are roughly triangular or bluntly relates to age and at this oblong with rounded umbones near (10cm) size the mussel is the anterior end. likely to be between 12 to The colour of the mussel (animal) itself 18 years old. Growth rates is a dark orange; the foot is red, whitish vary regionally. The shape towards the thick and wrinkled base. is an irregular oval, or Both margins of the mantle are rhomboid. It is bluish-white covered with delicate cilia to slate blue, darkening in The shell bears clear growth lines, and older specimens with a very a sculpture of fine concentric lines and glossy periostracum ridges. Numbers of growth bands can (chitinous layer), a light establish the growth rate, especially of horn colour to mahogany younger individuals. These rates vary a great deal with or dark brown in old shells, environmental circumstances. For example, ongoing studies of usually with a lighter both wild and enclosed mussels indicate lower growth rates for yellow-brown strip along mussels from deeper populations. the umbonal ridges. The strong, thread-like anchor Adaptations (byssal thread), allows the mussel Rapid growth is an to attach itself securely to almost adaptation to avoid any substrate, including each other. predation. Once a large size These tough threads are secreted as is reached growth slows a liquid by a gland near the foot. down. Individuals reaching This hardens upon contact with 45-60 mm in length are less water. In attaching to each other vulnerable to predation. they form protective clumps. These can grow to be large banks. Status/Threats Larvae and juveniles are Trawling and dredging destroy beds. heavily predated. Horse mussels have formed wave- Sightings in Nova Scotia like mounds in the Bay of Fundy up These are quite common in to 3 m high and 20 m wide and from 10 to 100 metres long. coastal areas.